Why you have to infuse 3 times the fluid loss with crystalloids?

1) Lets say someone lost 100 ml of blood due to accident. If you are giving crystalloid therapy, why do you have to infuse 3 times the amount.

What I think is when you give crystalloids IV, they go into extravascular and intravascular compartments also, not just stay in plasma . But my question is let's say I give 100 ml of crystalloid to this person, then wouldn't the patients body try to retain this 100 ml in plasma because now the hydrostatic pressures have changed due to the blood loss, shouldn't the body compensate and try to keep this in plasma. In that case why should you infuse 3 times the amount. In normal circumstances I can understand this fluid getting distributed in all the compartments.

2) Why exactly do crytalloid not stay in plasma when you infuse them.

*Do they move out due to normal hydrostatic presurre
*Or do they move out due to tonicity. More salt so water is sucked out from intracellular into plasma. Shouldn't this expand plasma volume.
* When they move out do they distribute in ICF and ECF like in normal water composition in the body, meaning 2/3 ICF and 1/3 ECF.

Volume share between the vascular and extracellular space. When you use crystalloids they leak into the extracellular space - and volume goes there too - so your vascular volume would drop again. Therefore, infude more volume than needed just to fill the vascular space.
Blood vessels are leaky and as a volume compartment the vascular and extracellular are not very distinct - they are somewhat continuous. Hence the composition of vascular and extracellular fluid is almost exactly the same bar free protein levels.

Volume share between the vascular and extracellular space. When you use crystalloids they leak into the extracellular space - and volume goes there too - so your vascular volume would drop again. Therefore, infude more volume than needed just to fill the vascular space.
Blood vessels are leaky and as a volume compartment the vascular and extracellular are not very distinct - they are somewhat continuous. Hence the composition of vascular and extracellular fluid is almost exactly the same bar free protein levels.

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Thank you for your answer But in a case of bleeding, shouldn't the bodies physiological mechanisms change, eg hydrostatic pressure decrease , and try to retain the given fluid in the plasma component?

* Also since normal saline is isotonic, does that mean normal saline is only distributed in extra cellular and plasma once given. Do they not go into cells? Does this mean hypotonic saline goes into cells.
*Also with hypertonic saline, why do plasma water go into ECF. Shouldn't it be other way round due to increased tonicity?
* Also how to colloids keep the water inside the plasma. Do collloids bind to water molecules and keep them?

Yes it will - but thats like saying we don't need to worry about plasma volume as long as vasoconstriction can compensate. That is good if it really needs to happen but not a good long term strategy. Thats why some volume sensors are not as closely linked to the idea of pressure, e.g. the atria and ANP. They are really more affected by volume delivered to the heart than arterial pressure - since that part of the circulatory system is so low pressure anyway.
Eventually the volume will equalize between all compartments yes (although osmolarities might have to shuffle around) - but there is a much more rapid equilibration between plasma and extracellular volumes.

Yes it will - but thats like saying we don't need to worry about plasma volume as long as vasoconstriction can compensate. That is good if it really needs to happen but not a good long term strategy. Thats why some volume sensors are not as closely linked to the idea of pressure, e.g. the atria and ANP. They are really more affected by volume delivered to the heart than arterial pressure - since that part of the circulatory system is so low pressure anyway.
Eventually the volume will equalize between all compartments yes (although osmolarities might have to shuffle around) - but there is a much more rapid equilibration between plasma and extracellular volumes.

* Also since normal saline is isotonic, does that mean normal saline is only distributed in extra cellular and plasma once given. Do they not go into cells? Does this mean hypotonic saline goes into cells.
*Also with hypertonic saline, why do plasma water go into ECF. Shouldn't it be other way round due to increased tonicity?
* Also how to colloids keep the water inside the plasma. Do collloids bind to water molecules and keep them?

Isotonic saline will quickly distribute to the vascular and extracellular space - and then it will slowly equilibrate with the intracellular space over time yes. Hypotonic saline will more rapidly bulk up intracellular volume due to the low tonicity and so during infusion will fill all compartments - vascular, extracellular and intracellular.
With hypertonic saline - you will incrase sodium concentrations in the vascular and extracellular space - remember think of those compartments as continuous as far as electrolytes are concerned. So water will come out of the cellular space and into the extracellular and vascular space.
Colloids are large molecules and unlike electrolytes do not generally travel well from the vascular to the extracellular space - therefore they tend to provide an osmotic force to keep fluid in the vascular space - which is why over perfusion of crystalloids dilutes out the natural plasma protein colloids and causes tissue edema - because you have lowered the osmotic 'draw' of water into the vascular space.

Isotonic saline will quickly distribute to the vascular and extracellular space - and then it will slowly equilibrate with the intracellular space over time yes. Hypotonic saline will more rapidly bulk up intracellular volume due to the low tonicity and so during infusion will fill all compartments - vascular, extracellular and intracellular.
With hypertonic saline - you will incrase sodium concentrations in the vascular and extracellular space - remember think of those compartments as continuous as far as electrolytes are concerned. So water will come out of the cellular space and into the extracellular and vascular space.
Colloids are large molecules and unlike electrolytes do not generally travel well from the vascular to the extracellular space - therefore they tend to provide an osmotic force to keep fluid in the vascular space - which is why over perfusion of crystalloids dilutes out the natural plasma protein colloids and causes tissue edema - because you have lowered the osmotic 'draw' of water into the vascular space.

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Thanks So hypertonic saline can be used to suck fluid from brain cells, when there is brain oderma. Also do colloids apply an oncotic pressure by binding to water molecules. Is their a chemical interaction with colloids and water, which prevents water molecules moving away.